1. Field of the Invention
The present invention relates to an exothermic electrically-conductive coating or paste and a heating unit incorporating the same, and more particularly to a heating unit having an exothermic, electrically-conductive coating which is operable at low voltages.
2. Related Art
Various conductive heating units, each of which comprises an electrically-conductive sheet or coating film containing a fine conductive powder and a binder, have previously been known, and in Japanese Patent Unexamined Publication Nos. 670-59131/1985 and 54- 149758/1979, a fine powder of carbon black, graphite, a metal or a metal oxide has been used as the fine conductive powder.
Prior art conductive heating units using carbon black or graphite as the fine conductive powder can be used at commercial voltages of 100 to 200 v, but can not be used at low voltages of 1.2 to 24 v which are applied, for example, when batteries are used as power sources, because the resistance of carbon black or graphite is too high to develop heat. For the purpose of reducing the resistance (.OMEGA.) of a conductive heating unit to between several tenths and several hundredths that of a heating unit containing carbon black or graphite, attention can be directed to the use of metal powders.
However, the metal powders incorporated in coating solutions, even if they are finely pulverized, precipitate during storage of the conductive coatings or during the solidifying process of the liquid coating films because of their high specific gravity. Consequently, it is difficult to obtain uniform exothermic surfaces in heating units by using the metal powders. One possible solution to prevent this precipitation is to mix the coating with a thickener or a suspending agent. However, such prior art thickeners or suspending agents can not effectively disperse the metal powders, and hence only exothermic, electrically-conductive coatings partially different in resistance are obtained. For this reason, such coatings are unsuitable for surface heaters which must have uniform temperature distribution.
At present, therefore, there are no surface heaters which can be operated at low voltages, such as when batteries are used as power sources.
Electrically-conductive compositions are known which incorporate fine conductive particles in a binder to provide high electrical conductivity. In the patent to Grindrup et al. (U.S. Pat. No. 4,624,798), for example, fine particles ("microballoons") of a low-density magnetic material are coated with a noble metal such as silver and dispersed in an electrically non-conductive matrix to provide a composite material claimed to be highly conductive electrically. Grindrup et al. specifically provides that the microballoons must be of magnetic material, and that it is the magnetic nature of the particles which imparts the high electrical conductivity to the composite material since the magnetic attraction of the microballoons, combined with their low density, permits the particles to be attracted to each other to form the network of metal-to-metal surface contacts which imparts the composite material with its high electrical conductivity. In the patent to Abrams (U.S. Pat. No. 4,419,279), a conductive paste is formed of inorganic non-metallic particles coated with silver metal and dispersed in a matrix of organic or glassy material, together with solid particles of silver metal. The paste is used to make electrically-conductive elements used in capacitors.
Compositions such as these are alleged to have high electrical conductivity, and are used in applications in which good electrical conductivity is important. Such materials, therefore, would not be suitable for use in an exothermic application wherein good electrical resistivity characteristics are more desirable.
The use of magnetic particles, such as in the Grindrup et al. patent, which may be desirable in a composition of improved electrical conductivity, has been determined by the present inventor to be undesirable in a material for use in exothermic devices and actually result in such devices with inferior performance characteristics. Even at room temperatures, but especially at high temperatures, magnetic hollow particles are readily oxidized and easily corroded. Thus, they become chemically unstable and the electrical resistance becomes unstable. If magnetic hollow particles were to be used, an exothermic heater device could not be made. Even if a heater device were possible with the use of hollow magnetic particles, because of their low resistance to oxidation and corrosion, the heater would perform unstably and quickly suffer quality changes in the normal operating temperature range for such devices of about 50.degree. to about 600.degree. C.